According to the prior art, automatic transmissions, particularly for motor vehicles, comprise planetary gear sets that are shifted using friction elements or shift elements such as clutches and brakes, and typically are connected to a start-up element, such as a hydrodynamic torque converter or a fluid coupling, that is subject to a slip effect and is provided optionally with a lock-up clutch.
Automatically shiftable vehicle transmissions of planetary design are already generally described numerous times in the prior art and are continually undergoing further development and improvement. These transmissions should have a relatively simple design, in particular requiring a low number of shift elements, and minimize the need for double shifting when sequential shifting is performed, that is, avoiding engaging or disengaging two shift elements, thereby ensuring that only one shift element is ever switched when shifting is performed in defined groups of gears.
The document DE 2721719 A1 describes a multistage transmission in planetary design having six forward gears and one reverse gear comprising three minus planetary gear sets, called the first, second and third planetary gear set in the following, disposed in a housing, six rotatable shafts, called the drive shaft, output shaft, third, fourth, fifth and sixth shaft in the following, and five shift elements. Here, the sun gear of the first planetary gear set is connected to the drive shaft which, via a first clutch, can be releasably connected to the sixth shaft connected to the sun gear of the second planetary gear set and to the sun gear of the third planetary gear set, and via a second clutch can be releasably connected to the fifth shaft, connected to the carrier of the second planetary gear set and the ring gear of the third planetary gear set, and which can be coupled via a third brake to the housing. With the known transmission it is also provided that the carrier of the first planetary gear set is connected to the fourth shaft, which is connected to the ring gear of the second planetary gear set and can be coupled via a second brake to the housing, and that the ring gear of the first planetary gear set is connected to the third shaft, which can be coupled via a first brake to the housing, wherein the output shaft is connected to the carrier of the third planetary gear set. The brakes and clutches of the transmission are implemented as friction engaged shift elements, particularly as multi-disk shift elements.
Because two engaged shift elements are required for each gear with the transmission according to the document DE 2721719 A1, with each gear, three friction engaged shift elements are disengaged, which disadvantageously results in undesired drag torques that negatively influence the efficiency of the transmission.
Further, with a 7 speed transmission proposed in the document DE 2721719 A1, it is provided that the first clutch is engaged for the first five forward gears, wherein the fourth brake only is required, and shifted into the power flow, for implementing the first forward gear and the reverse gear. This means that the first clutch and the fourth brake, because they are required for the first forward gear, are designed such that they support the entire engine torque including the maximum conversion. For the further gears of the transmission, a substantially smaller design of the shift elements would be sufficient.
Engaging a gear in the first forward gear, the coasting and tractive downshift from second gear into the first forward gear, and the coasting and tractive downshift from sixth gear into the fifth forward gear, are qualitatively negatively influenced due to the maximum dimensioning of the first clutch and the fourth brake. In order to optimize the shift quality with these shifts, the number of disks for the first clutch and the third brake is reduced according to the prior art, which however disadvantageously results in reduced transfer capability of these shift elements in the first forward gear.
From the prior art, for example from the documents DE 10 2008 000 429 A1 and DE 10 2007 022 776 A1 by the applicant, transmissions in planetary design are known, in which a portion of the shift elements are implemented as form-locking shift elements.
Due to the design of a portion of the shift elements of a transmission as form-locking shift elements, the power loss due to the drag torque of disengaged shift elements is reduced, and the transfer capability is increased with respect to the shift elements, wherein the mechanical overall gear ratio spread remains the same.
The mechanical overall gear ratio spread of a transmission is a key control variable for operating the upstream internal combustion engine at an optimal operating point, whereby the fuel consumption can be reduced. Further, the mechanical overall gear ratio spread of a transmission is an important parameter in order to attain specific driving performance in special applications.
Further developments of existing transmissions known from the prior art, implemented to be shiftable under load, result in a slight increase of the overall gear ratio spread and are disadvantageously complex and expensive.
Further, from the prior art it is known to combine automatic transmissions with additional automatically shiftable distributor transmissions, which are integrated in the drive strategy, for representing a group shift, thereby increasing the mechanical overall gear ratio spread.
However, this design has the disadvantage that a two-stage distributor transmission is necessary for representing a group shift, which results in high manufacturing and assembly costs and large construction space needs. The distributor transmission, as a rule, is implemented as a transmission in countershaft design. Additionally, the distributor transmission, with respect to the attainable transmission ratio, the design of the form-locking shift elements and the synchronization measures, must be adapted to the upstream transmission and the internal combustion engine. In addition, the group shifts disadvantageously cannot be shifted under load.